According to the Biot–Savart law, the magnitude of the field dB produced by a current element I dl at distance r, where θ is the angle between dl and r, is:

1. Biot–Savart law in vector form: $d\mathbf{B} = \dfrac{\mu_0}{4\pi}\dfrac{I\,d\mathbf{l}\times\mathbf{r}}{r^3}$. 2. The magnitude of $d\mathbf{l}\times\mathbf{r}$ is $dl\, r\sin\theta$. 3. So $|d\mathbf{B}| = \dfrac{\mu_0}{4\pi}\dfrac{I\,dl\,r\sin\theta}{r^3} = \dfrac{\mu_0}{4\pi}\dfrac{I\,dl\sin\theta}{r^2}$. 4. The inverse-square dependence on $r$ rules out the $1/r$ option. _Source: syllabus Physics Ch 4 "Moving Charges and Magnetism", p.143_